Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition, overlay and other parameters of nanoscale structures.
In some examples, optical critical dimension (CD) and film metrologies (spectroscopic or angle-resolved) are employed to monitor structural parameter values during manufacture to ensure that structures are fabricated having the desired pitch and profile. Often, however, the measurements of structures at different locations are performed separately. In other words, information gleaned from the measurement of a structure at one location is not utilized in the measurement of a repeated instance of the structure at another location. In addition, model-based metrology techniques include modeling errors induced by practical, computational limitations. For example, a number of parameters are fixed in a model-based measurement, so that regression calculations remain tractable. Hence, not all parameters are floated during computation, and this leads to errors in the estimates of parameter values.
Future metrology applications present challenges for metrology due to increasingly small resolution requirements, multi-parameter correlation, increasingly complex geometric structures, and increasing use of opaque materials. Thus, methods and systems for improved measurements are desired.